Opaque films whose thickness is from 10 to 500 μm are known.
EP-A-1 125 967 likewise describes white opaque films comprised of crystallizable thermoplastics. However, that specification says nothing about the dielectric strength or the density of the films produced. Furthermore, those films comprise only barium sulfate and an optical brightener. In all of the examples listed, the transparency of those films is >12%, and is therefore unsuitable for the application sectors demanded here.
U.S. Pat. No. 4,780,402 describes white films with barium sulfate, and optical brightener if appropriate, and, if appropriate, incompatible polymer for adjustment of transparency. The density of all of those films is in the range from 1.1 to 1.33 kg/dm3. This low density indicates a very high number of voids (vacuoles), leading to markedly impaired dielectric strength. Furthermore, the yellowness index of those films is in the negative range, i.e. the film has a blue tinge.